1. Field of the Invention
The present invention relates to particulate concentrate compositions formed by intercalation of a polymer polymerizing component into the galleries of a layered inorganic cation exchange composition initially in proton exchanged form and to the use of the particulate concentrates for the preparation of cured polymer-inorganic nanolayer hybrid composite compositions. In the most preferred embodiment of the invention the layered inorganic composition is selected from the family of 2:1 layered silicate cation exchangers.
2. Description of the Related Art
Organic-inorganic hybrid composites can exhibit mechanical properties superior to those of their separate components. To optimize the performance properties of these materials, it is usually desirable to disperse the inorganic components in the organic matrix on a nanometer length scale (Giannelis, E. P. JOM 44 28 (1992); Gleiter, H. Adv. Mater. 4 474 (1992); and Novak, B. M., Adv. Mater. 5 422 (1993)). Smectite clays and other layered inorganic materials that can be broken down into nanoscale building blocks (Pinnavaia, T. J. Science 220 365 (1983)) are useful for the preparation of organic-inorganic nanocomposites.
U.S. Pat. Nos. 3,432,370 to Bash et al; 3,511,725 to Stevens et al, 3,847,726 to Becker et al and Canadian Patent No. 1,004,859 to Nelson show various compositions incorporating flexible epoxy resins. There are numerous uses for these polymer matrices.
Smectite clays are natural or synthetic layered aluminosilicate such as montmorillonite, bentonite, hectorite, fluorohectorite, saponite, beidellite, nontronite, and related analogs. Smectite clays have layered lattice structures in which the tactoids (crystallites) consist of stacked two-dimensional oxyanions separated by layers of hydrated cations. The oxygen atoms define layers approximately 10 .ANG.-thick, containing two sheets of tetrahedral sites and a central sheet of octahedral sites. The 2:1 relation between the tetrahedral and the octahedral sheets in a layer defines 2:1 layered silicates. For a typical 2:1 layered silicate, such as montmorillonite, the layer is made up of a central octahedral sheet, usually occupied by aluminum or magnesium, sandwiched between two sheets of tetrahedral silicon sites. Various isomorphous cation substitutions, e.g., Si.sup.4+ by Al.sup.3+ in the tetrahedral sheet, or Al.sup.3+ by Mg.sup.2+, or Mg.sup.2+ by Li.sup.+ in the octahedral sheet, among others, also result in negatively charged nanolayers. These negatively charged layers are approximately 10 .ANG.-thick, and are separated by hydrated cations such as alkali or alkaline earth metal ions in the gallery (interlayer) regions between the 2:1 layered silicates. The negative charge on the layer is balanced by interlayer or "gallery" cations, normally Ca.sup.2+ and Na.sup.+. The gallery cations in a natural smectite can be replaced by simple ion exchange process with almost any desired cation, including alkylammonium alkyl phosphonium and other organic cations. Some idealized unit cell compositions and layer charge densities of smectite clays are listed in Table 1.
TABLE 1 ______________________________________ Ideal Structural Formulas for some 2:1 Layered Silicates. Layer Charge Density Mineral per O.sub.20 Name Ideal Formula unit ______________________________________ Hectorite M.sub.x/n.sup.n+.yH.sub.2 O[Al.sub.6.0-x Mg.sub.x ](Si.sub.8.0)O.s ub.20 (OH).sub.4 0.4-1.2 Fluoro- M.sub.x/n.sup.n+.yH.sub.2 O[Al.sub.6.0-x Mg.sub.x ](Si.sub.8.0)O.s ub.20 (OH,F).sub.4 0.4-1.2 hectorite Mont- M.sub.x/n.sup.n+.yH.sub.2 O[Mg.sub.6.0-x Li.sub.x ](Si.sub.8.0)O.s ub.20 (OH).sub.4 0.6-1.0 morillonite Nontronite M.sub.x/n.sup.n+.yH.sub.2 O[Fe.sub.4.0 ](Si.sub.8.0-x Al.sub.x)O.s ub.20 (OH).sub.4 0.6-1.0 Beidellite M.sub.x/n.sup.n+.yH.sub.2 O[Al.sub.4.0 ](Si.sub.8.0-x Al.sub.x)O.s ub.20 (OH).sub.4 0.8-1.0 Saponite M.sub.x/n.sup.n+.yH.sub.2 O[Mg.sub.6.0 ](Si.sub.8.0-x Al.sub.x)O.s ub.20 (OH).sub.4 0.6-1.2 Vermiculite Mg.sub.(x-z)/2.sup.2+ [Mg.sub.6-z Fe.sup.III.sub.z ](Si.sub.8-x Al.sub.x)O.sub.20 (OH).sub.4 1.2-1.4 Muscovite K.sub.2 [Al.sub.4.0 ](Si.sub.6.0 Al.sub.2.0)O.sub.20 (OH).sub.4 2.0 mica Biotite mica K.sub.2 [Al.sub.y Mg.sub.6+(x/2)-(3y-2) ](Si.sub.6.0-x Al.sub.2.0+ x)O.sub.20 (OH).sub.4 2.0 (x &lt; 1, y &lt; 2) Phlogopite K.sub.2 [Mg.sub.6.0 ](Si.sub.6.0 Al.sub.2.0)O.sub.20 (OH).sub.4 2.0 mica ______________________________________
Included in Table 1 for comparison purpose are the idealized compositions of 2:1 layered silicates, smectite clays, vermiculite, muscovite mica, biotite mica, and phlogopite mica. Vermiculite has a layer charge density higher than a smectite but lower than a mica. Micas usually have layer charge of 2.0. The gallery cations in a vermiculite or a mica can also be replaced by ion exchange, but the exchange processes are generally slower than for smectite clays. Also, vermiculites and micas exist commonly as single crystals that range in size from 10 .mu.m to 10 cm or larger. In contrast, smectite clays have sub-micron particle sizes. The particle size of vermiculite and mica can be reduced to the micron size range by mechanical grinding. Other ion exchangeable 2:1 layered silicate including illite, rectorite and synthetic derivative such as tetrasilicic mica and synthetic mica montmorillonite (SMM).
Those skilled in the art will know that smectites are members of a more universal class of layered inorganic ion exchangers. Many other layered inorganic cation exchanger materials can be selected in place of smectites. These layered materials include crystalline sheet silicate, layered phosphates, arsenates, sulfates, titanates and niobates.
The crystalline sheet silicates include kenyaite: Na.sub.2 Si.sub.20 O.sub.41.xH.sub.2 O, x=10; magadite: Na.sub.2 Si.sub.20 O.sub.41.xH.sub.2 O, x=3; makatite: Na.sub.2 Si.sub.4 O.sub.9.xH.sub.2 H, x=3; kanemite: NaHSi.sub.2 O.sub.5.xH.sub.2 O, x=3, revdite: Na.sub.2 Si.sub.2 O.sub.5.5H.sub.2 O; Grumantite: NaHSi.sub.2 O.sub.5.xH.sub.2 O, x=0.9, and Ilerite Na.sub.2 Si.sub.8 O.sub.17.
The layered phosphates, arsenates, titanates and niobates are listed as follows:
TABLE 2 ______________________________________ Class Compound general formula ______________________________________ Phosphates H.sub.2 {M.sup.IV (PO.sub.4).sub.2 }.xH.sub.2 O, (M.sup.IV = Zr, Ti, Ge, Sn, Pb) CaPO.sub.4 R.H.sub.2 O (R = CH.sub.3, C.sub.2 H.sub.5), VOPO.sub.4.2H.sub.2 O, NbOPO.sub.4.3H.sub.2 O, H{SnCl(OH)PO.sub.4 }.2H.sub.2 O Arsenates H.sub.2 {M.sup.IV (As.sub.4).sub.2 }.xH.sub.2 O, H{MnAsO.sub.4 }.H.sub.2 O (krautite), H{SnCl(OH)AsO.sub.4 }.2H.sub.2 O Titanates Na.sub.2 Ti.sub.3 O.sub.7,K.sub.2 Ti.sub.4 O.sub.9, Na.sub.4 Ti.sub.9 O.sub.20.xH.sub.2 O, K.sub.2 Ln.sub.2 Ti.sub.3 O.sub.10.H.sub.2 O Vanadates KV.sub.3 O.sub.8 Niobates KTINbO.sub.5,CSTi.sub.2 NbO.sub.7, A.sub.3 Ti.sub.5 NbO.sub.14, (A = Li, Na, K, Rb, Cs, Tl), KNb.sub.3 O.sub.8, K.sub.4 Nb.sub.6 O.sub.17, ACa.sub.2 Nb.sub.3 O.sub.10, (A = K, Rb, Cs) Molybdates MoO.sub.3 (OH), H.sub.x MoO.sub.3 Uranyl H{UO.sub.2 PO.sub.4}.4H.sub.2 O, H{UO.sub.2 AsO.sub.4 56 .4H.sub.2 O Compound Manganates Busertite ______________________________________
Most important among the properties of smectite clays is the ability to replace the gallery cations in the pristine clay with almost any desired cations by ion exchange reactions. The exchange cations are very important in determining the ability of the gallery regions to imbibe (intercalate) neutral molecules. Inorganic cations (M.sup.n+) such as (Na.sup.+, Ca.sup.2+ and the like) prefer to be solvated by polar molecules such as water and certain polar organic molecules. Thus, these exchange forms are readily swollen by polar molecules, especially by water. Organic cations (alkylammonium, phosphonium ions and the like) are less hydrophilic, even hydrophobic, and prefer to intercalate organic molecules into the gallery regions. Inorganic cations such as K.sup.+ and Mg.sup.2+ in mica are anhydrous and strongly bound to the intergallery surfaces. Therefore, these silicates are difficult for gallery swelling and ion exchange reaction. However, the exchange of gallery cations in micas can be facilitated by reducing the particle size of the particles, preferably to average values of 2 .mu.m or less.
Clay-organic intercalates are intercalation compounds in which organic molecules enter the clay galleries and form definite compositions with specific clay basal spacings. The organic compounds that have been reported to form clay intercalates include uncharged polar organic compounds and positively charged organic compounds. These two classes of guest species are intercalated by ion exchange, physical adsorption, or other mechanisms. Intercalation of organic polymers in clay minerals has been recognized to occur as natural processes in soils. Polymer adsorption and desorption also occurs under synthetic conditions (Theng, B. K. G. "The Chemistry of Clay-Organic Reactions", John Wiley & Sons, pages 136 to 206 (1974)). Interaction between clays and polymeric species have been discussed as natural or synthetic polymer adsorption and desorption (Theng, B. K. G. "Formation and Properties of Clay-Polymer Complexes", Elsevier pages 63 to 133 (1979)).
In general, the polymer-clay composites can be divided into three categories: conventional composites, intercalated nanocomposites, and exfoliated nanocomposites. In a conventional composite, the clay tactoids exist in their original state of aggregated layers with no intercalation of the polymer matrix between the layers of the clay. The polymer contacts the external surfaces of the clay particles (tactoids) exclusively. In an intercalated nanocomposite the insertion of polymer into the clay layer structure occurs in a crystallographically regular fashion, regardless of the clay-to-polymer ratio. An intercalated nanocomposite normally is interlayered by only a few molecular layers of polymer and the properties of the composite typically resemble those of the ceramic host (Kato, C., et al., Clays Clay Miner. 27 129 (1979); Sugahara, Y., et al., J. Ceram. Soc. Jpn. 100 413 (1992); Vaia, R. A., et al., Chem. Mater. 5 1694 (1993); and Messersmith, P. B., et al., Chem. Mater. 5 1064 (1993)). In contrast, in an exfoliated nanocomposite, the individual 10-.ANG.-thick clay layers are separated in a continuous polymer matrix by average distances that depend on loading. Usually, the clay content of an exfoliated clay composite is much lower than that of an intercalated nanocomposite. Consequently, an exfoliated nanocomposite has a monolithic structure with properties related primarily to those of the starting polymer.
The exfoliation of smectite clays provides 10 .ANG.-thick silicate layers with high in-plane bond strength and aspect ratios comparable to those found for fiber-reinforced polymer composites. Exfoliated clay nanocomposites formed between organocation exchanged montmorillonites and thermoplastic nylon-6 have recently been described (Fukushima, Y., et al., J. Inclusion Phenom. 5 473 (1987); Fukushima, Y., et al., Clay Miner. 23 27 (1988); and Usuki, A., et al., J. Mater. Res. 8 1179 (1993); and WO 93/04117 and 93/04118 describing thermoplastic polymers). Clay exfoliation in the nylon-6 matrix gave rise to greatly improved mechanical, thermal, and Theological properties, making possible new materials applications of this polymer (Usuki, A., et al., J. Mater. Res. 8 1179 (1993); and Kojima, Y., et al., J. Mater. Res. 8 1185 (1993)). Recently clay-reinforced epoxy nanocomposites have been reported (Lan, T. and Pinnavaia, T. J. , Chem. Mater. 6 2216 (1994)) by using alkylammonium exchanged smectite clays in a flexible epoxy matrix. The reinforcement of the exfoliated 10-.ANG.-thick clay layers was very significant. For instance, 15 wt % of the CH.sub.3 (CH.sub.2).sub.17 NH.sub.3.sup.+ -montmorillonite in the epoxy matrix increased the tensile strength 10 times and modulus 8 times. The significant reinforcing benefit provided by the silicate was especially significant for a flexible matrix. U.S. Pat. No. 4,889,885 to Usuki et al shows thermoplastic vinyl polymer composites containing clay.
For all the polymer-clay nanocomposites reported to date, alkylammonium onium ions, or .alpha.,.omega.-amino acid ions were exchanged into the clay galleries prior to nanocomposite formation, in part, to make the galleries more hydrophobic and better suited for interaction of polymer precursors. These organoclays allow intercalation (access) of monomer species (e.g., .epsilon.-caprolactam, epoxy resin and curing agent) into the clay gallery (Usuki, A., et al., J. Mater. Res. 8 1179 (1993), Lan, T. and Pinnavaia, T. J. , Chem. Mater. 6 2216 (1994)). Upon polymerization reaction, the monomers form a network in the clay gallery regions and a polymer-clay nanocomposite is formed. By controlling the intra- and extragallery polymerization rate of the monomers, exfoliated and intercalated nanocomposites can be prepared.
Alkylammonium exchanged clays also have been used to form polymer-clay compositions by direct polymer melt intercalation (Vaia et al. Chem. Mater., 7 154, (1995)). The process involves heating a polymer-silicate mixture either statically or under shear in an extruder above the softening temperature of the polymer.
In the previous art, the presence of long chain alkyl onium ions in the clay galleries was essential for allowing the monomer or the pre-formed polymer to migrate into the clay gallery. However, the alkylammonium ions in the gallery can block potentially favorable van der Waals interactions of the polymer matrix with the clay gallery surfaces. Also, the high cost of the alkylammonium ions and complex processing procedures limit the applications of the composites. Furthermore, the alkylammonium ions are toxic and require special handling procedures. Thus, eliminating the need for alkylammonium exchange cations in forming polymer-inorganic nanolayer composites would be a great practical and economical benefit.
Another problem restricting the use and performance properties of polymer-inorganic nanolayer hybrid composites is the difficulty in forming the composites with the inorganic nanolayers in the preferred exfoliated state. The prior art teaches two general ways of achieving inorganic nanolayer exfoliation in a polymer matrix. One approach is to form the polymer from monomeric polymer precursors or mixtures of polymer precursors in the presence of a layered inorganic ion exchanger interlayered by organic onium ions. However, in many cases the polymerization rate for polymer formation is much slower in the interlayer gallery region of the layered inorganic phase than in the bulk polymer. Consequently, intercalated rather than exfoliated hybrid nanocomposites are formed. Also, this "in situ" polymerization strategy lacks manufacturing versatility in the production of parts with hybrid nanocomposite compositions, because the nanocomposites can only be produced in batches of fixed polymer to inorganic nanolayer ratio.
The second approach to nanocomposite formation mixes a pre-formed thermoplastic polymer with the layered inorganic ion exchanger, typically modified with alkylammonium exchange ions. Melt processing the mixture under applied sheer in an extruder can lead to nanocomposite formation under suitable circumstances. But melt processing is limited to thermoplastics with melting temperatures below the decomposition temperature of the onium exchange cation.
In view of the above limitations of the prior art, more versatile processing compositions and processing methods applicable to both thermoset and thermoplastic polymers are needed in order to more efficiently manufacture a broader range of polymer-inorganic nanolayer hybrid composite compositions.